Design of compact multi-band microstrip bandpass filter having simultaneously excited passbands by using open-circuited stubs

In this paper, a novel compact multi-band bandpass filter design is realized by using a microstrip meander square loop resonator and open-circuited stubs which are located at 0°-90°-180°-270° according to the excitation ports. The proposed resonator structure can be used to design wideband bandstop filter with two notch bands or quad-band bandpass filter by connecting input/output ports to the resonator directly or capacitively coupled, respectively. In addition, quad-band bandpass filter includes two dual-mode passbands and two ultranarrow single mode passbands. Degenerate modes of two dualmode passbands can be simultaneously excited using only one conventional perturbation element in the orthogonal corner. To demonstrate the validity of proposed configuration and design procedure, a quad-band bandpass filter was designed, fabricated and tested. It can be used for the communication systems having the operating frequencies of 1.78, 2.3, 3.11 and 3.45 GHz. Measured results are in a very good agreement with the simulated results.